We propose experiments to measure local sensitivity and local rhodopsin concentration in isolated retinal rods and cones of tiger salamander employing extra-cellular suction electrodes and microspectrophotometry, respectively. Both measurements will be made on the same cell with the same optical system. Specific questions are: 1. Are the effects of bleaching adaptation free to spread longitudinally in rod outer segments, or are they confined to areas of the outer segment containing discs with bleached rhodopsin? 2. Are the effects of bleaching adaptation free to spread longitudinally in cone outer segments? Can time dependent spread of bleaching adaptation be observed which is consistent with longitudinal spread of bleached pigment within the lipid matrix of the membrane? 3. What is the extent of the endogenous pool of 11-cis retinoid in isolated rods and cones from which rhodopsin can be regenerated in darkness following rhodopsin bleaching? 4. Can exogenous application of 11-cis retinoids promote recovery of sensitivity and regeneration of rhodopsin in these cells? 5. Can bleaching adaptation reverse low calcium induced desensitization in rods and cones in the same way background light does? Recent experiments have shown that we can apply patch clamp techniques to enzymatically isolated photoreceptor cells from the retina of the scallop, Lima scabra. We propose the following experiments to examine the effects of excitation and adaptation in single channel and macroscopic membrane currents in these cells: 1. What is the elementary nature (i.e. single channel conductance, ion selectivity, gating, kinetics, etc.) of light and non-light induced currents of which the light induced receptor response is composed? 2. What are the effects of light adaptation on the time, light, and voltage dependence of these single ion channels? 3. What is the nature and extent of spatial spread of excitation and adaptation in these cells?